The present invention relates to a multilayer polymeric film having dead bend characteristics. More particularly, the present invention relates to a multilayer polymeric film comprising at least two layers of one or more polymeric films, each of which is highly oriented uniaxially. Such multilayer polymeric film possesses dead bend characteristics regardless of the direction of the fold in the plane of the film.
In general, food wraps (used herein to refer only to sheet goods, thereby excluding closable bags and similar containers) are of three general types: aluminum foil, waxed paper, and clear plastic (e.g., thermoplastic polymer) films. Waxed paper must be taped or otherwise restrained in a closed condition. Plastic films generally remain closed to a greater or lesser extent because of the tendency of the film to adhere weakly to itself and, in some instances, to a container. Of the three types of wraps, only aluminum foil remains in a closed condition because it possesses dead bend characteristics. However, only the plastic films are transparent, an obviously desirable feature.
As used herein, the term "dead bend" refers to the ability of a film to remain in a deformed state, which ability results from the lack of elastic or viscoelastic recovery from deformation. The deformation encountered in the use of a food wrap typically is bending and rarely tensile.
To obtain a dead bend plastic or polymeric film, it is postulated that the elastic limit of the polymer must be lowered approximately one order of magnitude and that time dependent relaxation must be essentially eliminated. It further is postulated that such goals perhaps can be achieved by preparing films from highly oriented polymers. However, films prepared from highly oriented polymers typically fibrillate readily in the direction of orientation.
Highly oriented films and fibers are, of course, well known. In many cases, the high degree of orientation is achieved by stretching the material to a draw ratio of the order of about 20 or, in some cases, higher. By way of illustration, a number of representative references are described in the paragraphs which follow.
The preparation of high-strength polyethylene continuous filaments is described by W. Wu and W. B. Black, Polm. Eng. Sci., 19, 1163 (1979). Six high density polyethylenes were employed in the study. In some cases, draw ratios as high as about 32 were possible. Reportedly, fibers with high breaking strength as well as high Young's modulus were made from polymers having a number average molecular weight greater than 22,000 by way of a spinning and drawing process which met the following criteria: (a) an extrusion temperature above 250 degrees C; (b) quenching of the molten filament in air under a certain amount of tension; and (c) a drawing temperature from 120 to 130 degrees C.
Morphology and tensile property relations of high-strength/high-modulus polyethylene fibers are described by W. Wu et al., J. Polym. Sci., Polym. Phys. Ed., 18, 751 (1980). The fibers studied apparently were prepared by high-temperature extrusion followed by hot drawing at draw ratios up to 25. The study also included the use of different types of polyethylene.
A high-modulus shaped article is described in British Provisional Specification, Application No. 9796/1974, filed Mar. 5, 1974 on behalf of G. Capaccio and I. M. Ward, which article may be obtained by subjecting a crystallizable, essentially linear organic polymer having a selected molecular weight distribution to a controlled thermal treatment whereby substantially complete alignment of its molecules is obtained when the polymer undergoes attenuation at an imposed rate and temperature. Suitable treatments are stated to involve (1) slowly cooling the polymer from a temperature close to its melting point, optionally quenching the polymer; (2) quickly cooling the polymer and then holding it at a fixed temperature for a period of time to allow crystallization to occur, optionally followed by quenching; or (3) very rapidly cooling the polymer to a temperature where crystallization occurs only very slowly, optionally followed by reheating. The acceptable rates of cooling in (1), (2), and (3) and the time and temperature in (2) depend upon the type of polymer used and its molecular weight. In the examples, draw ratios ranged from 12 to 54.
The above reference appears to be but one example of a continuing research effort by Giancarlo Capaccio and/or Ian MacMillan Ward, often with coworkers, relating to high-strength fibers and films. See also, by way of illustration: G. Capaccio and I. M. Ward, Nature (London), Phys. Sci., 243, 143 (1973) [Chem. Abstr., 79:79380m (1973)]; G.. Capaccio and I. M. Ward, Polymer, 15, 233 (1974) [Chem. Abstr., 81:121128d (1974)]; German Offenlegungschrift No. 2,410,747, filed Sept. 12, 1974 on behalf of I. M. Ward and G. Capaccio [Chem. Abstr., 82:P17519s (1975)]; French Demande No. 2,234,982, filed Jan. 24, 1975 on behalf of I. M. Ward and A. G. Gibson [Chem. Abstr., 83:P98428g (1975) ]; G. Capaccio and I. M. Ward, Polymer, 16, 239 (1975) [Chem. Abstr., 83:115135d (1975)]; G. Capaccio et al., Polymer, 16, 469 (1975) [Chem. Abstr., 83:147892q (1975)]; G. Capaccio and I. M. Ward, Poly. Eng. Sci., 15, 219 (1975) [Chem. Abstr., 83:28932t (1975)]; G. Capaccio et al., J. Polym. Sci., Polym. Phys. Ed., 14, 1641 (1976) [Chem. Abstr., 85:160721u (1976)]; G. Capaccio et al., Polymer, 17, 644 (1976) [Chem. Abstr., 85:178100a (1976)]; French Demande No. 2,330,716, filed June 3, 1977 on behalf of G. Capaccio and I. M. Ward [Chem. Abstr., 88:P5l539g (1978)]; G. Capaccio and I. M. Ward, Polymer, 18, 967 (1977) [Chem. Abstr., 88:170596h (1978)]; British Patent No. 1,498,628, issued on Jan. 25, 1978 and filed on Oct. 3, 1973 on behalf of G. Capaccio and I. M. Ward [Chem. Abstr., 89:90786a (1978)]; G. Capaccio et al., Polym. Eng. Sci., 18, 533 (1978) [Chem. Abstr., 88:170918q (1978)]; G. Capaccio et al., Ultra-High Modulus Polym., [Lect. Semin.], 1977 (Published 1979) [Chem. Abstr., 92:598l8a (1980)]; G. Capaccio et al., J. Polym. Sci., Polym. Phys. Ed., 18, 301 (1980) [Chem. Abstr., 92:129782k (1980)]; G. Capaccio, Atti Conv.-Sc. Crist. Polim., 1979, 260 [Chem. Abstr., 95:62903s (1981)]; G. Capaccio et al., J. Polym. Sci., Polym. Phys. Ed., 19, 1435 (1981) [Chem. Abstr., 95:133551p (1981)]; and G. Capaccio, Macromol. Chem. Phys., Suppl., 4, 197 (1981) [Chem. Abstr., 94:157319z (1981)].
U.S. Pat. No. 4,053,270 to J. R. Collier and T. Y. T. Tam describes an apparatus for the extrusion of highly oriented polymeric materials. The apparatus appears to be generally conventional, except that the axial orifices extending through the extrusion die are longer than those of conventional dies. The longer length orifices reportedly induce molecular orientation in the direction of flow within the molten polymeric material being extruded therethrough. Temperature control of the system is reported to be critical.
A process for producing high tenacity polyethylene fibers is reported in U.S. Pat. No. 4,228,118 to W. Wu and W. B. Black and appears to be based upon the work described in Wu and Black, supra. According to the patent, polyethylene yarns having tenacities of at least 12 grams per denier are produced at commercially feasible spinning speeds by a process in which a high density polyethylene having a number average molecular weight of at least 20,000 and a weight average molecular weight of less than 125,000 is extruded through a spinneret at a temperature of from 220 to 335 degrees C to form yarns which are hot-drawn at a temperature between about 115 and 132 degrees C. Draw ratios of from about 20 to about 35 or higher apparently can be employed.
Of course, a high degree of orientation in fibers and films can be achieved by means other than stretching already formed, solidified materials. For example, one class of highly oriented substances consists of liquid crystals which may be polymeric. See, e.g., Martin Grayson, Executive Editor, "Kirk-Othmer Encyclopedia of Chemical Technology," Third Edition, Vol. 14, John Wiley & Sons, New York, 1981, pp. 395-427; W. J. Jackson and H. F. Kuhfuss, J. Polym. Sci., Polym. Chem., 14, 2043 (1976); U.S. Pat. No. 3,671,542 to S. L. Kwolek; U.S. Pat. No. 4,067,852 to G. W. Calundann; U.S. Pat. No. 4,118,372 to J. R. Schaefgen; and U.S. Pat. No. 4,181,792 to W. J. Jackson and H. F. Kuhfuss.
Other methods for obtaining highly oriented fibers and films include gel-drawing and hydrostatic extrusion; see, by way of illustration only, P. Smith and P. J. Lemstra, Makromol. Chem., 180, 2983 (1979) and Polymer, 21, 1341 (1980); U.S. Pat. No. 4,356,138 to S. Kavesh et al.; J. H. Southern and R. S. Porter, J. Appl. Polym. Sci., 14, 2305 (1970); and E. S. Clark and L. S. Scott, Polym. Eng. Sci., 14, 682 (1974).
As an example of gel-drawing, U.S. Pat. No. 4,413,110 to S. Kavesh and D. C. Prevorcek may be mentioned. This patent describes high tenacity, high modulus polyethylene and polypropylene fibers and intermediates thereof. Solutions of ultrahigh molecular weight polymers such as polyethylene in a relatively nonvolatile solvent are extruded through an aperture, at constant concentration through the aperture, and cooled to form a first gel of indefinite length. The first gel is extracted with a volatile solvent to form a second gel and the second gel is dried to form a low porosity xerogel. The first gel, second gel, or xerogel, or a combination thereof, then are stretched. In the examples, stretching was accomplished at draw ratios up to about 175.
Multilayer films, of course, are well known. For a general discussion of some physical properties of such films, see, by way of illustration only, W. J. Schrenk and T. Alfrey, Jr., Polym. Eng. Sci., 9, 393 (1969). Several somewhat more specific references are described briefly below.
U.S. Pat. No. 3,017,302 to P. B. Hultkrans describes improvements in the use of composite heat-shrinkable wrappers for packaging commodities. The wrapper apparently consists of a layer of a heat-shrinkable film, such as a polyester which has been stretch oriented to render it heat shrinkable, and either a second film laminated thereto or a coating thereon, which second film or coating consists of a thermoplastic material which is heat sealable, such as polyethylene.
U.S. Pat. No. 3,342,657 to G. B. Dyer relates to a process and apparatus for producing a laminated oriented thermoplastic film. The process comprises extruding a tube of thermoplastic material in its formative state, passing the tube over a cooled internal mandrel to cool the tube to a temperature which is below the temperature of the thermoplastic material in its formative state, maintaining sufficient pressure within the tube to at least prevent collapse of the tube, advancing the tube at a predetermined initial rate, heating the tube to the orientation temperature range by passing the tube over a heated internal mandrel, pulling the tube at a rate of from 2 to 5.5 times the initial rate to longitudinally uniaxially orient the tube, and cooling the tube to a temperature which is below the temperature of the material in the formative state by passing the tube about a cooled mandrel. If desired, the tube can be extruded from a rotating annular die and the extruded tube rotated throughout the remainder of the process, followed by slitting the rotating tube to strip form along substantially helical lines after the orientation and cooling steps to give a uniaxially oriented continuous strip. Subsequent stretching of the strip will yield a biaxially oriented film. Two such uniaxially oriented strips can be laminated together in such a way that the direction of orientation of one strip is substantially at right angles to the direction of orientation of the other strip.
A method for producing and orienting polypropylene films is described in U.S. Pat. No. 3,380,868 to R. Moser. Briefly, two or more unoriented polypropylene films having thicknesses of less than about 10 mils (about 250 microns) are secured together by bringing the films into intimate contact while simultaneously heating them at a temperature sufficient to promote good surface contact. The resulting film structure then is oriented, preferably by stretching first in a longitudinal direction and then in a transverse direction, optionally followed by another stretching in the longitudinal direction. Draw ratios apparently are in the range of from about 0.2 to 3.
U.S. Pat. No. 3,539,439 to G. C. Calderwood and D. Poller relates to polyolefin laminates which are heat sealable. According to the patent, polypropylene or ethylene-propylene copolymers can be processed to form heat-sealable films. Such films are formed by extruding or casting predominantly crystalline polymer into sheet stock of suitable thickness. The cast polymer then is extrusion coated with a polymer of a free-radical polymerizable monomer, the polymer having a lower softening temperature than the sheet stock polymer. The sheet stock may be coated on one or both sides. The coated sheet stock then is biaxially oriented by stretching laterally and longitudinally at draw ratios of from about 4 to about 8.
U.S. Pat. No. 3,547,768 to R. E. Layne describes a heat-shrinkable laminate. The laminate is stated to comprise two webs of plastic film having a very thin layer of barrier wax distributed therebetween. The laminate is produced by integrating laminating and stretching operations in such a manner that a layer of molten barrier wax is disposed between two films which then are stretched while the wax is in the molten condition. Although the plastic films can be made from a variety of polymers, the preferred polymer apparently is poly(vinyl chloride).
U.S. Pat. No. 4,022,646 to K. Casey describes a process for the co-orientation lamination of ethylene polymer films. The process comprises the steps of (1) bringing together in face-to-face engagement at least one ethylene polymer film (A) with at least one ethylene polymer film (B) and (2) while maintaining the films in face-to-face engagement, orienting the films at a temperature below the melting point of the polymer of film (A), the polymer of film (A) being of higher melting point than the polymer of film (B). Orientation apparently is carried out uniaxially at draw ratios of from 1.5 to 7.
A method for producing a laminated high-strength sheet is described by U.S. Pat. No. 4,039,364 to O. Rasmussen. According to the patent, the method comprises the steps of attenuating while extruding each of at least two layers of at least one molten polymer blend to impart to the polymer a unidirectional grain with a pronounced direction of splittability when solidified, before or after solidification of said layers uniting the layers into a common sheet with the grain direction of adjacent layers therein extending in criss-crossing relationship, while forming a generally weak bond between said layers, solidifying said layers if not already solid, and finally biaxially orienting the solid laminated sheet thus obtained in several steps which each are generally substantially uniaxial at a temperature sufficiently low for maintaining a significant splittability in each layer. In order to allow a local delamination during tearing and thereby make the tear "fork", it is essential to form a generally weak bond in the lamination of the layers. Stretch ratios appear to be less than 3.
U.S. Pat. No. 4,337,285 to M. Akao and K. Kashiwagi describes a wrapping material for light-sensitive materials. The material is obtained by bonding at least two uniaxially drawn films composed of high density polyethylene having a density of at least 0.94 g/cm.sup.3 with an adhesive layer containing an aluminum paste or an aluminum powder so that the drawing axes thereof cross each other at an angle of from 45 to 90 degrees. Draw ratios can range from 2 to 6.
A coextruded, heat-shrinkable, multilayer polyolefin packaging film is the subject of U.S. Pat. No. 4,352,849 to W. B. Mueller. The film comprises an ethylene-vinyl acetate copolymer layer sandwiched between two layers of a copolymer of propylene. The multilayer film is oriented so that it is heat shrinkable in at least one direction, the preferred stretching ratios being from 4 to 7 in each of the transverse and longitudinal directions.
Finally, U.S. Pat. No. 4,374,690 to P. J. Canterino and C. E. Allen describes multidirectionally oriented films. The multidirectional orientation is achieved by using grooved rollers which impart a random orientation to the film as it passes between such grooved rollers, with the path of orientation varying sinusoidally. The random orientation of the film is directly related to the roller diameter and the helical angle, i.e., the angle between the axis of the groove and the axis of the roller. Two films having such multidirectional orientation can be laminated together for increased strength, tear resistance, and stiffness. While not mentioned, draw ratios inherently must be low.
In spite of all of the work with highly oriented fibers and films and the efforts directed at providing improved plastic or polymeric wrapping materials, there clearly is a need for a food wrap which combines the most desirable properties of plastic or polymeric films and aluminum foil, i.e., a clear or translucent plastic or polymeric food wrap which exhibits dead bend characteristics. Such a wrap can be used in microwave ovens, whereas aluminum foil cannot. Furthermore, many polymeric films exhibit excellent barrier properties.